1. Field of the Invention
The present invention relates to an optical recording medium equipped with a recording layer that performs the recording and playback of information by irradiation with a laser beam or the like, and to a method for manufacturing the medium, a method for recording and playback with the medium, and an apparatus for recording and playback with the medium.
2. Background Information
Optical recording media have been drawing attention as a large-capacity, high-density memory. Such media come in an erasable type that allows the rewriting of information, and write once type that allows information to be recorded only once. One such erasable optical recording medium has as its recording layer a thin film on a substrate, and this film undergoes phase changes between amorphous and crystalline. The recording and playback of information is performed by means of thermal energy produced by irradiation with a laser beam.
Known phase changing materials that have been used for this recording layer include alloy films whose main component is germanium, antimony, tellurium, indium, or the like, such as a GeSbTe alloy. The recording of information is accomplished by forming marks through the partial amorphization of the recording layer, while erasure is performed by crystallizing these amorphous marks. The amorphization is performed by first heating the recording layer to its melting point or over, and then cooling it at a specific rate or faster. Crystallization, meanwhile, is performed by heating the recording layer to its crystallization temperature or over but below its melting point. The playback of information is performed by utilizing the difference in reflectance between these amorphous marks and the crystalline regions.
The general practice is to provide a substrate with spiral or concentric guide grooves for tracking a laser beam during recording and playback. The areas between the grooves are called lands. The information track is where information is recorded in either the grooves or on the lands, and whichever of these grooves and lands that is not used as the information track usually serves as a guard band for separating adjacent information tracks. For instance, with a Blu-ray Disc, information is recorded in the groove part of the information track, which is closer to the laser beam irradiation side.
FIG. 7 shows a conventional example of an optical recording medium in which information is recorded in the grooves. In FIG. 7, an optical recording medium 701 comprises a recording layer on a transparent substrate with a thickness of 1.1 mm, composed of a polycarbonate, and having in its center a center hole 702 for placing the disk in a recording and playback apparatus. A protective layer with a thickness of 0.1 mm is further provided over the recording layer. The recording and playback of information is performed by applying a laser beam through the protective layer. The substrate is provided with guide grooves for tracking the laser beam during recording and playback. The optical recording medium 701 also has a lead-in zone 704 used solely for playback, a test recording zone 705 where a learning operation is performed for determining the optimal pulse conditions, and an information recording zone 706 in which information is recorded.
More recently, the quantity of information handled has risen along with the processing capabilities of various kinds of information devices. Accordingly, there has been a need for a recording medium that is less expensive and affords higher density. An example of such a recording medium is a write once type of recording medium in which the recording layer is an organic dye film formed by spin coating.
FIG. 4a is a cross section of the main components in the radial direction of a write once type of recording medium, while FIG. 4b shows an example of forming a recording layer by spin coating. A recording medium 401 is created by forming a recording layer 433 over a substrate 436 having a thickness of 1.1 mm and equipped with an information track, and then forming a transparent protective layer 432 over this in a thickness of 0.1 mm. A laser beam 410 is applied through the protective layer 432 to perform the recording and playback of information. The recording layer 433 is formed by coating the substrate 436 with an organic dye 449 while rotating the substrate 436, which is disposed on a rotary table 448.
Here, the recording layer 433 is thicker in the concave lands 445 of the information track, and is thinner in the convex grooves 446, so signal quality is not high enough when information is recorded in the grooves 446.
In view of this, it is possible to improve signal quality by recording information in the concave lands 445. However, it is a problem for recording media with which information is recorded in the grooves 446 to co-exist with recording media with which information is recorded in the lands 445, as it becomes difficult to accommodate this situation on the recording and playback apparatus side.
It is an object of the present invention to solve the above problems encountered in the past, and to provide an optical recording medium that allows information to be recorded and played back with good signal quality for different types of recording medium, and to provide a method for manufacturing this medium, a method for recording and playback with this medium, and an apparatus for recording and playback with this medium.